Shaft furnace operation with a double fuel injection

ABSTRACT

A fluid hydrocarbon is injected into the lower portion of a furnace, by means of a burner. The temperature of the combustion gas, resulting from the burning of the hydrocarbon and any coke in the furnace, is at least 1,600*C. Reducing gas is injected into the upper portion of the furnace at the level of the lower part of the reserve zone. The temperature of the reducing gas as it enters the furnace is equal to or slightly greater than the temperature of the material in the furnace at the level of injection of the reducing gas.

United States Patent Poos et al.

[ SHAFT FURNACE OPERATION WITH A DOUBLE FUEL INJECTION [75] Inventors: Arthur Gerard Poos, Embourg;

Nicolas Gerassimos Ponghis, Liege, both of Belgium 22] Filed: 0a. 12, 1971 21 Appl.No.: 188,013

[ Sept. 24, 1974 2,799,576 7/1957 Gumz et al. 75/41 FOREIGN PATENTS OR APPLICATIONS 945,312 12/1963 Great Britain 75/42 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-M. J. Andrews Attorney, Agent, or Firm-l-l0lman & Stern [5 7 ABSTRACT A fluid hydrocarbon is injected into the lower portion of a furnace, by means of a burner. The temperature of the combustion gas, resulting from the burning of the hydrocarbon and any coke in the furnace, is at [52] U.S. Cl. 75/42 least C- Reducing gas is injected into the upper [51] Int. Cl C21b 5/00 portion of the furnace at h l l f the lower part of [58] Field of Search 75/41, 42 the reserve zone- Th t mp ratur of the reducing gas as it enters the furnace is equal to or slightly greater [56] Ref r Cit d than the temperature of the material in the furnace at UNITED STATES PATENTS the level of injection of the reducing gas.

1,393,749 10/1921 Carstens 75/42 16 Claims, 2 Drawing Figures HE IBHl l 1 NEEUPERANUN ZUNE 1 u z I E 1 5 i 1 RESERVE ZDNE 1 l l z 5| Hm REDUCING 01s I I lNJECllllN LEVEL ELABURAlIIJN ZllNE MAIN IUYERES CUMBUSTIUN 0F HYDRUEARBUN BY MEANS llFlJXY-FUEL BURNERS PATENTEDSEPZMSH SHEUlUF 2 FIG. I

SHAFT FURNACE OPERATION WITH A DOUBLE FUEL INJECTION BACKGROUND OF THE INVENTION The present invention relates to improvements in methods for operating shaft-type furnaces, particularly blast furnaces in which ferriferous substances are processed.

For some years now, steelmakers have been endevoring to reduce the coke rate, which is the amount of coke necessary to produce one ton of pig iron.

PRIOR ART Among the processes at present employed for this purpose, the injection through the main tuyeres of auxiliary fuels such as natural gas, fuel oil, etc., has already made it possible to reduce the coke rate by an amount which at present is approximately 50 kg/t(metric ton) pig iron.

When users tried to increase the amount of fuel oil injected through the tuyeres in order to reduce the coke rate still further, they found that the result was either too great a reduction in the flame temperature at the tuyere outlet, so that the furnace did not operate satisfactorily, or a waste of fuel oil since it was not all burnt, or a combination of these two disadvantages. Two measures were employed in an attempt to remedy the situation, viz, increasing the blast temperature and increasing the oxygen content of the blast, and by this means it was possible, on an industrial scale, to effect a saving of the order of 150 kg normal coke pig iron. It is difficult to increase this saving further for technical reasons connected with the first measure (the increasing of blast temperature). Furthermore, the second measure (increasing the oxygen content of the blast) cannot be pursued above a certain level since the heat content of the rising gases is then no longer enough to meet the heat requirements of the top portion of the furnace, viz. to heat the solid material as it falls. This situation is indicated particularly by an excessive reduction in the temperature of the top gas.

In order to remedy these two new disadvantages, it has been proposed that hot reducing gases be injected at the level of the reserve zone, with the essential effect of this second injection operation being to re-establish the heat balance in the furnace and to create particularly favorable conditions in which the preliminary reduction of the burden fed in at the top of the furnace may take place.

It should be added that another limitation of the injection of substantial quantities of fuel at tuyere level has to be reckoned with, even when the above described double injection process is used. This limitation is the result of the devices generally used for this injection, whose design does not permit a thorough mixing of the fuel and the combustion-supporting gas so that the proportion of unburnt substances which pass into the top gas scrubbing water particularly is not economically tolerable.

OBJECTS AND SUMMARY OF THE INVENTION When experimenting with the simultaneous double injection process described above, we discovered the operating conditions required for this process to be fully effective, and the subject of the invention relates to these optimum operating conditions for the double injection process.

In what concerns the lower injection process, which is effected at the lower portion of the furnace and preferably at the level of the tuyeres, the temperature of the gases resulting from the combustion of the hydrocarbon injected and of a certain quantity of coke at the injection location, must be compatible with furnace operation and reach at least l,600C and preferably at least l,800C. Moreover, the combustion of the hydrocarbon injected must be carried out with a minimum of unburnt product or soot.

What is desired is a suitable means of meeting these requirements, which means consists in utilizing for the lower injection operation, an oxygen burner known as an oxyfuel burner which has the following advantages over the devices generally used:

a. it is easy to obtain a high temperature of combustion gases at the tuyere outlets;

b. the fuel and the combustion supporting gas are mixed thoroughly together, so that the amount of unburnt material is reduced to a minimum; and

c. it is simple to adjust the temperature and the composition of the gases at injection level by regulating the amount of oxygen.

In what concerns the upper injection process, which has hitherto been little used, the features contributed by the present invention are at least as important and relate both to the location of the injection and the temperature of the gases injected.

We have discovered that for maximum effectiveness, this injection should be performed in the vicinity of the lower portion of the reserve zone, at a point where the solid materials are at a temperature of between 900 and 1,100C. It was found that the injection of reducing gases loses its effectiveness as it moves upwards from the lower portion of the reserve zone.

A second feature of this upper injection of reducing gases, usually containing mainly CO and H is that their temperature as they enter the furnace should be equal to or slightly higher than the temperature of the solid materials at the injection location.

On the basis of these considerations, the process constituting the object of the invention, in which a hydrocarbon fluid is injected (lower injection) at the lower portion of the furnace and preferably at the level of the main tuyeres, while reducing gases (upper injection) are injected at the level of the reserve zone, is essentially characterized in that the lower injection is effected by means of at least one oxygen burner, known as an oxy-fuel burner, the temperature of the gases resulting from the combustion of the said hydrocarbon fluid and possibly also of a certain quantity of coke at the location of injection being at least 1,600C and preferably at least 1,800C, with this injection possibly being effected without the addition of the air generally blown in at this location and in that the upper injection of reducing gases containing mainly CO and H is effected in the vicinity of the lower portion of the reserve zone, the temperature of the reducing gases at the point where they enter the furnace being equal to or slightly higher than the temperature of the material at the injection location.

By hydrocarbon fluid should be understood any gaseous hydrocarbon fuel, or one in a liquid or flowable semi-solid state, such as natural gas and the various types of fuel oil.

The oxy-fuel burner used for the lower injection process may be used with a slight excess of oxygen in relation to the stoichiometric proportion giving the combustion of the hydrocarbon fluid to produce CO and H the proportion of excess oxygen varying with the type of hydrocarbon fluid used and making it possible, with by combustion of a small quantity of hot coke at the tuyere outlets, easily to produce gases at the high temperature required. In the case of light hydrocarbons, such as CH or natural gas, the excess oxygen should be greater than in the case of heavy hydrocarbons such as heavy fuel oil, with the amount of heat generated by the combustion of these light hydrocarbons being substantially lower and having to be compensated by the combustion of a greater amount of coke.

Hence, one advantageous feature of the method of the invention consists in regulating the temperature of the gases at the lower injection location by regulating the flow of oxygen to the oxy-fuel burner. This regulation consisting of simple adjustments to a flow of gas (oxygen) is particularly simple to carry out and may easily be automated.

The oxy-fuel burners used may be introduced into the conventional blast tuyeres, with the outlets of the burners being preferably located substantially in line with the outlets of the tuyeres.

The hydrocaron fluid and/or oxygen used for the lower injection process may advantageously be preheated in order to facilitate the achieving of the high temperatures desired. This preheating may, in particular, be important when the hydrocarbon used is (ll-l or natural gas.

The quantity of reducing gases introduced into the furnace by the upper injection process should preferably be sufficient to meet the heat and reduction requirements of the upper zone. It is advantageously such that at least 70 percent of the iron in a metal state in the burden is collected at the lower portion of the reserve zone.

Moreover, these reducing gases which are used for the upper injection process may be brought to the temperature required by any conventional heating means, such as a heat accumulator.

The temperature of the gases introduced into the furnace by the upper injection process may be controlled by a suitable means as a function of the temperature of the rising gases at the injection location, so as to compensate for the fluctuations in the temperature of the materials at this point. It is possible, for example, to use gases, whose temperature at the outlet of the injection device is l,300C and to regulate their temperature at the entry to the furnace by adding a cold gas, preferably a reducing gas. This control may be effected automatically by any suitable means, such as by regulating the addition of cold gas by linking it automatically to the temperature of the rising gases at the injection location.

Also in what concerns the upper injection process, the hot reducing gases may be reformed gases obtained by well known techniques of oxidizing conversion and- /or the thermal cracking of a hydrocarbon fluid, or again may be obtained from blast furnace gases and/or coke oven gas after suitable treatment by conventional means.

The hot'reducing gases injected at the upper level of the shaft-type furnace are advantageously produced in a suitable reactor and conveyed to the injection points chosen, while observing the necessary conditions of temperature and flow imposed by the heat requirements of the part of the shaft furnace in which injection is effected. To this end. the reactors are connected to the appropriate injection points by a circular duct or ring which is suitably heatinsulated by refractory material or the like.

The present invention also has as its object to define the optimum design of an oxygen burner used for effecting this injection into a shaft furnace.

A burner of this kind is elongated in shape and preferably has a circular cross-section which permits of easy and rapid location and removal.

It is closed adjacent to the outlet of the two fuels by a plate having at least one aperture for fuel and at least one for combustion-supporting gas. These apertures constitute the respective ends of the circuits supplying the fuel and gas to the burner, with these circuits being formed, along the full length of the burner, by two independent ducts. The circuits are equipped with a cooling system in which water circulates so as to enclose as completely as possible each of the burner feed circuits and to cool them effectively.

The internal cross-section of the outlet apertures for the fuel and gas is such that they mix thoroughly together at a given distance from the burner end-plate, and this distance normally is of the order of 3 to 25 cm and preferably between 6 and 13 cm.

Consequently the burner of the invention, which is used for the injection of a hydrocarbon fluid into a shaft-type furnace and in particular into a blast furnace in which ferriferous materials are treated, comprises:

a. an external enclosure of elongated shape and preferably of circular cross-section, which allows the burner to be positioned and removed easily and rapidly,

b. a plate which closes the end of the enclosure at the side adjacent to the fuel outlet, the plate being provided with at least one aperture for the fuel, and at least one aperture for combustion-supporting gas,

c. at least one fuel duct which passes through the external enclosure along its full length and which is connected to the outlet aperture(s) for said fuel, and at least one duct for the gas, which also passes through the external enclosure along its full length and which is connected to the aperture(s) for the gas,

d. an outlet aperture or aperture for the gas and for the fuel, having an internal cross-section such that the jets of each are mixed thoroughly at a given distance from the plate having the apertures, and

e. an inlet aperture to the enclosure and an outlet aperture which are designed to permit circulation of cooling water throughout the inside volume of the enclosure, with this water enclosing the fuel and gas ducts, so as to ensure effective cooling of the burner.

The plate comprising the outlet apertures for the gas and the fuel may be perpendicular or oblique to the longitudinal axis of the burner, depending upon its position and the direction in which is to be imparted to the injected substances.

According to another constructional feature, the outlet aperture or apertures for the fuel are located in the central region of the plate which closes the correspond ing end of the burner, and the outlet apertures for the combustion-supporting gas are located about the former.

In order to permit supervision of operation, the burner is provided with a sight hole which takes the form of a passage free of any obstruction, which passes through the burner along its full length and terminates in a hole provided in the fuel outlet plate.

The combustion-supporting gas may, of course, be

oxygen, which is preferably commercially pure, or a gas rich in oxygen. The fuel may be gaseous, such as natural gas or liquified gas, or extra-heavy or semi-solid fuel oil in the form of bitumen or tar.

In the case of the liquid fuel, the mixture of the jets of the fuel and combustion-supporting gas has the additional effect of causing atomization of the liquid fuel.

Another object of the present invention consists in defining the optimum construction and arrangement of tuyeres specially designed to receive the oxygen burners when these burners are not arranged in the main tuyeres.

A tuyere of this type is particularly provided with an external enclosure which is preferably symmetrical and round in particular, with the length thereof being at least equal to the thickness of the lining of the furnace at the point of injection while the internal cross-section is basically designed to permit the burner to be installed and removed rapidly.

This tuyere must also be provided with a device for centering the burner relatively to the longitudinal axis of the tuyere, and the outlet line of the burner gases preferably coincide with said longitudinal axisof the tuyere.

Finally, this tuyere must also be provided with a burner fixing device which nonetheless allows the burner to be released rapidly when necessary.

Consequently the injection tuyere of the present invention comprises:

a. an external enclosure which is preferably symmetrical and round in particular, the length thereof being at least equal to the thickness of the lining of the furnace at the point of injection, while the internal crosssection is sufficiently free to allow the burner to be installed and removed rapidly.

b. a device for centering the burner relatively to the longitudinal axis of the tuyere, with the outlet line of the burner gases preferably coinciding with the longitudinal axis of the tuyere,

c. a device for fixing the burner in operating position which allows of rapid release when necessary.

Advantageously, it may also comprise a slideway which allows the burner to move in parallel with the longitudinal axis of the tuyere so as to be able to adjust the distance between the outlet of the burner and that of the tuyere.

The external enclosure of the tuyere of the invention comprises an internal cooling water circuit and is advantageously constituted, at least at the end directed towards the inside of the furnace, of a metal having very good heat conductiveproperties, such as very pure copper, e.g., electrolytic copper.

Also according to the invention, the tuyere is advantageously provided with a transparent screen which forms a sight hole through which the progress of operations may be supervised.

In the case of tuyeres specially designed for the burner and when no air is blown in, the centering device is advantageously constituted by a suitably tapered end-piece, the taper being turned towards the inside of the furnace and also allowing direct contact between the burner and the end-piece, so that there is an excellent coefficient of heat exchange between these two elements. This end-piece is preferably shaped like a truncated cone but it may also be provided with a spherical surface which serves as a coupling with the burner.

This end-piece is, of course, advantageously provided with an internal cooling water circuit and made of a metal having very good heat conductive properties.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described further, by way of example only with reference to the accompanying drawing, in which drawing:

FIG. 1 shows part of a blast furnace in vertical section, and

FIG. 2 is a diagrammatic view illustrating the various zones of the furnace.

DETAILED DESCRIPTION OF THE INVENTION A shaft 1 of the furnace has a refractory lining 2, protected on the outside by plating 3. The frusto-conical external enclosure of the injection tuyere comprises and end-piece 4 and an extension 5.

The frusto-conical end-piece 4 is made of very pure copper, such as electrolytic copper, in order to ensure the optimum heat dissipation, since this part is that most exposed to thermal stresses. This tuyere is obviously effectively cooled by an internal water circuit.

This end-piece also acts in this embodiment as the device which centers a burner 6 within the injection tuyere, owing to its conical shape and the large surface of contact 7 between the burner 6 and end-piece 4.

The extension 5 acts particularly as a means of fixing the tuyere in the lining 2 of the furnace and to the external plating 3. This extension may be made in one piece or in two pieces. In the latter case, the different parts generally known as casing, tymp and nozzle in the main tuyeres are found here.

FIG. 1 also shows a sight hole 8 through which progress of operations may be followed. The burner 6 is secured to the plating 3 by connections 9 and 10 which may be removed quickly and easily.

The oxy-fuel burner 6 is lodged within the external enclosure 4, 5 and centered by means of a frustoconical end-piece 4. The large surface of contact between the bumer 6 and the frusto-conical end-piece 4 allows a substantial heat exchange to take place so that the burner 6 is effectively cooled.

The oxy-fuel burner 6 is, of course, provided with separate ducts for the hydrocarbon and oxygen fluids which have not been shown in order not to confuse the drawing. The burner 6 is also provided with a free passage which serves as a sight hole and one end of this passage terminates at 8.

A device of this type thus has the great advantage of being locatable and removable rapidly with a minimum of simple manipulations. Moreover, the mechanical and thennal protection of the burner is very effective. Finally, the provision of a sight hole makes it possible to follow the progress of operations.

The method described above provides an effective method of markedly reducing the coke consumption in shaft-type furnaces to a point where, in principle, the only coke consumption would be a complement required for the carbonization of the iron and the coke necessary for the reduction of the Si, P, and Mn.

In FIG. 2, the profile of the blast furnace with the upper and lower injections can be seen in the left portion of the figure. The curve in dash lines illustrates the variation of the mean temperature inside the furnace in function of the height of the furnace. The temperatures are reported along the axis of the abscissae with a minimum at the top of the furnace and a maximum at the level of the lower injection.

In the right portion of the figure are denoted the different zones of the furnace, with the location of the upper injection, hot reducing gas injection level, and of the lower injection, main tuyeres: combination of a by drocarbon with an oxy-fuel burner.

During research, tests were carried out on an experimental furnace with the double injection of reformed gas in the manner described above.

A fluid hydrocarbon is injected into the lower portion of the furnace, by means of a burner. The temperature of the combustion gas, resulting from the burning of the hydrocarbon and any coke in the furnace, is at least l,600C. Reducing gas is injected into the upper portion of the furnace at the level of the lower part of the reserve zone. The temperature of the reducing gas as it enters the furnace is equal to or slightly greater than the temperature of the material in the furnace at the level of injection of the reducing gas.

The following table shows the characteristics of this furnace in normal operation, i.e., without injections (control), and then with injections (test). It should be pointed out that the results thus obtained are necessarily lower than the results which would be obtained on a normal-capacity industrial furnace in which heat losses are much lower than in an experimental furnace.

2,300C. These gases are sufficient to meet the heat requirements in the lower zone of the furnace.

' c. An additional 700 m (normalized) of reformed gas is injected. after preheating to 1,00()C, into the upper portion of the furnace, in order to meet the heat requirements and provide the necessary reducing elements in the shaft.

Procedure 2 a. 185 kg metallurgical coke is used per metric ton of pig iron.

b. At tuyere level 210 m (normalized) natural gas is burnt per ton of pig iron, using oxy-fuel type burners with partial combustion in the presence of O and the combustion gas are injected.

c. At the higher level reducing gas containing mainly CO and H is injected in a quantity of the order of 770 m (normalized) per metric ton of pig iron, these gases being preheated to a temperature of l,OOOC.

The term material used in the claims may be the solid mixture of iron ores, coke and fluxes which goes down through the furnace from the stock lines or the gas which goes up through the furnace in counter current to the solids.

We claim:

1. A method of operating a shaft furnace for reducing iron oxides and producing pig iron in which the furnace includes an upper zone in which a cold and oxidized burden enters and from which leave products preheated at about 1000C and pre-reduced to wustite state, a lower zone in which the preceding pre-heated and pre-reduced burden enters and from which leave liquid pig iron and slag and a reserve zone located in the lower part of the upper zone wherein no thermal exchange and no chemical reaction occurs, comprising the steps of: injecting a fluid hydrocarbon into the TAB EE Characteristics Control Test Lower injection light fuel oil kg/t i iron I63 oxygen N'nh lt pigiroii [92 Upper injection reformed gas Nm lt pig iron 925 Blast Temperature C 1020 920 Oxygen 2! 26 I Production(pig iron) t/h 1.24 1.84

normal coke rate kg/t pig iron 662 392 dry coke rate kg/t pig iron 648 382 S till within the scope of the method of the present in vention, we may recommend the following procedure where it is desired to produce a pig iron containing 4.5 percent C and 1:2 percent Si, which would need a flame temperature at the tuyeres of 2,300C: Procedure I a. Coke used is of the order of 125 kg/t pig iron; part of this, viz 44 kg, is burnt at the tuyere outlet at a flame temperature of 4,000C.

b. 220 kg fuel oil (extra heavy) is used to effect partial combustion in the oxy-fuel burners located at the level of the tuyeres. This fuel oil is burnt in the presence of oxygen and gives combustion gases at a temperature of the order of 2,000C. The result at the level of the tuyeres is an injection of 700 m (normalized) of reducing gases at a temperature of approximately lower zone of the furnace by means of at least one oxyfuel burner, the temperature of the resultant combustion gas being at least 1,600C. as it enters the furnace and simultaneously injecting reducing gas into the upper zone at the level of the lower part of the reserve zone, the temperature of the reducing gas as it enters the furnace being substantially equal to the temperature of the material within the furnace at the level of injection of the reducing gas.

2. The method as claimed in claim 1, wherein the combustion gas temperature is at least 1,800C.

3. The method as claimed in claim 1, wherein the reducing gas is mainly a mixture of CO and H 4. The method as claimed in claim 1, including at least one oxy-fuel burner so that the lower injection process is carried out with a slight excess of oxygen in relation to the stoichiometric proportion allowing CO and H to be produced by the combustion of the hydrocarbon fluid.

5. The method as claimed in claim 4, further comprising regulating the flow of oxygen to the oxy-fuel burner thus regulating the temperature of the combustion gas.

6. The method as claimed in claim 1, providing oxyfuel burners within conventional main blast tuyeres.

7. The method as claimed in claim 1, including preheating the hydrocarbon fluid or oxygen used for the lower injection.

8. The method as claimed in claim 1, comprising introducing reducing gas in a quantity sufficient to satisfy the thermal and reduction requirements of the upper zone into the furnace by the means of upper injection.

the furnace at the injection location.

11. The method as claimed in claim 10, wherein the temperature of the reducing gas before injection is slightly higher than the temperature required, further comprising adding a cold gas to the reducing gas to regulate the temperature of the reducing gas as it is injected into the furnace.

12. The method as claimed in claim 10, comprising controlling automatically the temperature of the reducing gas introduced into the furnace by means of the upper injection as a function of the temperature of the rising gases at the injection location.

13. The method as claimed in claim 1, wherein the hot reducing gas comprises reformed gas.

14. The method as claimed in claim 1, wherein the hot reducing gas is obtained at least in part from blast furnace gas or coke oven gas.

15. The method as claimed in claim 1, comprising producing reducing gas in a suitable reactor and conveying the reducing gas to the injection points chosen.

insulated annular duct. 

2. The method as claimed in claim 1, wherein the combustion gas temperature is at least 1,800*C.
 3. The method as claimed in claim 1, wherein the reducing gas is mainly a mixture of CO and H2.
 4. The method as claimed in claim 1, including at least one oxy-fuel burner so that the lower injection process is carried out with a slight excess of oxygen in relation to the stoichiometric proportion allowing CO and H2 to be produced by the combustion of the hydrocarbon fluid.
 5. The method as claimed in claim 4, further comprising regulating the flow of oxygen to the oxy-fuel burner thus regulating the temperature of the combustion gas.
 6. The method as claimed in claim 1, providing oxy-fuel burners within conventional main blast tuyeres.
 7. The method as claimed in claim 1, including pre-heating the hydrocarbon fluid or oxygen used for the lower injection.
 8. The method as claimed in claim 1, comprising introducing reducing gas in a quantity sufficient to satisfy the thermal and reduction requirements of the upper zone into the furnace by the means of upper injection.
 9. The method as claimed in claim 8, comprising introducing into the furnace by means of upper injection, a quantity of reducing gases such that at least 70 percent of the iron in a metal state in the burden is collected at the lower portion of the reserve zone.
 10. The method as claimed in claim 1, comprising controlling the temperature of the reducing gas introduced into the furnace by the upper injection process as a function of the temperature of the gases rising in the furnace At the injection location.
 11. The method as claimed in claim 10, wherein the temperature of the reducing gas before injection is slightly higher than the temperature required, further comprising adding a cold gas to the reducing gas to regulate the temperature of the reducing gas as it is injected into the furnace.
 12. The method as claimed in claim 10, comprising controlling automatically the temperature of the reducing gas introduced into the furnace by means of the upper injection as a function of the temperature of the rising gases at the injection location.
 13. The method as claimed in claim 1, wherein the hot reducing gas comprises reformed gas.
 14. The method as claimed in claim 1, wherein the hot reducing gas is obtained at least in part from blast furnace gas or coke oven gas.
 15. The method as claimed in claim 1, comprising producing reducing gas in a suitable reactor and conveying the reducing gas to the injection points chosen.
 16. The method as claimed in claim 15, wherein the reactor is connected to the injection points by a heat-insulated annular duct. 